1. Field of the Invention
The present invention relates to infrared detectors. More specifically, the present invention relates to readout electronics used in focal plane arrays of infrared detectors.
2. Description of the Related Art
Arrays of detectors are often used in imaging applications. These arrays are disposed in the focal plane of the imaging system and are therefore known in the art as focal plane arrays. Focal plane arrays (FPAs) are particularly useful in infrared imaging applications. For certain applications, small detector pitch (e.g. 25 microns per side) is a critical design parameter. The pitch of the detector relates to the size thereof and allows for high-resolution imaging.
In accordance with conventional teachings, focal plane arrays are typically constructed with on-chip signal processing and contained within the space of the detector area. This has heretofore limited the extent to which the detector could be reduced in size. FPAs with smaller pitch have been proposed, however, these designs force either the analog signal processing outside the array or force the degradation of the resolution achievable for the signal processing contained in the area defined by the detector pitch. Thus, such functions as high speed sampling of the received signals and digitization of same could not be achieved as these functions required a large number of sample and hold (S/H) operations which can not be implemented within the detector pitch.
Hence, the prior art in small pitch focal plane array design suffered from an inability to perform on-chip sample and holds and was limited to simply reading out the received/processed signals in a row/column multiplexed (MUX) fashion on a single signal line. This prior approach was an integration based approach and was generally limited in its ability to perform high resolution signal processing. That is, to achieve small pitch, the intensity of the received waveform was integrated. However, this approach was limited in its ability to detect multiple returns. Further, this prior approach was typically constructed using silicon detectors, mercury-cadmium-telluride detectors or avalanche photodiodes connected to an integrator. The integrators are then multiplexed to a digital signal processor via an analog to digital converter. Unfortunately, this system only outputs intensity data. It provides no indication of is multiple returns, the rise and fall times, the peak intensity (the shape of the received waveform) or the location of the peaks therein.
In short, readout electronics was limited to simple signal processing due to the requirement of maintaining signal processing circuitry inside the unit cell. The output of the circuitry was typically multiplexed either along a column or a row in order to reduce the routing required outside the cell. Multiplexing is slow and does not allow parallel signal processing of the detector outputs.
Hence, there is a need in the art for a system or method for achieving a high density of small detector pitch readout unit cells which allow for high resolution on-chip analog signal processing of a received waveform in a focal plane array.
The need in the art is addressed by the unit cell of the present invention. The inventive unit cell includes a substrate; an active circuit disposed on the substrate; and an arrangement disposed on the substrate for routing a plurality of conductors thereover. In the illustrative implementation, the routing arrangement includes first, second and third ground planes disposed on the substrate, a first layer of conductors disposed between the first and second planes, and a second layer of conductors disposed between the second and the third planes. Each cell is adapted to connect to a device such as a detector.
The inventive unit cell enables an improved focal plane array design with a smaller unit cell supporting smaller detector sizes. Smaller detector pitch allows higher density detector arrays. The inventive fan-out approach allows for complicated circuitry to be located outside the array. This permits the utilization of more sophisticated analog signal processing, such as the multiple sample approach. Multiple sampling results in a much more accurate digital Gaussian curvefit, which increases the range and intensity accuracy of readout electronics.